High Strength Spring Steel Wire and High Strength Spring and Methods of Production of the Same

ABSTRACT

The present invention provides high strength spring and high strength spring steel wire superior in corrosion fatigue characteristics and methods of production of the same, that is, a high strength spring steel wire and high strength spring containing, by mass %, C: 0.35 to 0.50%, Si: 1.00 to 3.00%, and Mn: 0.10 to 2.00%, restricting P to 0.015% or less and S to 0.015% or less, having a balance of Fe and unavoidable impurities, and, when raising the temperature in the range from 50° C. to 600° C. by 0.25° C./s and measuring the differential scanning calories, having the only peak of the exothermic reaction present at 450° C. or more. A method of production of high strength spring characterized by tempering under conditions where the tempering temperature T[K], tempering time t[s], and content Si % [mass %] of Si satisfy the following: 
       16000≦(T−40×[Si %])×(31.7+log t)≦23000.

TECHNICAL FIELD

The present invention relates to a high strength spring preferable for asuspension spring of an automobile etc., a material for the same, thatis, a high strength spring steel wire, and methods of production of thesame.

BACKGROUND ART

Due to the demands for lightening the weight of auto parts, suspensionsprings and the like are being asked to be raised in strength. Animportant issue in raising the strength of suspension springs isimprovement of the corrosion fatigue characteristics. Suspension springsare painted for use, but pebbles etc. bounce against them while theautomobile is moving, the parts of the springs contact each other, etc.resulting in unavoidable peeling of the paint, so corrosion and pittingare unavoidable. Fatigue cracks occur in the suspension springs startingfrom the corrosion pits due to such pitting, so technology for adjustingthe ingredients of the springs and spring use steel wire to suppresscorrosion pits is being reported (for example, Nakayama, Takenori etal., “Corrosion Fatigue Characteristics of High Strength SuspensionSpring Steel and Improvement of Same”, Kobe Steel Technical Reports,vol. 47, no. 2, July 1997, issued by Kobe Steel, p. 50 to 53; Kimura,Kazuyoshi et al., “Effects of Alloy Elements on Corrosion Fatigue Lifeof Spring Steel”, Electric Furnace Steel, vol. 75, no. 1, January 2004,Electric Furnace Steel Research Group, p. 19 to 25; Kurebayashi, Yutakaand Yoneguchi, Akio, “1200 MPa Class High Strength Spring Steel‘ND120S’”, Electric Furnace Steel, vol. 71, no. 1, January 2000,Electric Furnace Steel Research Group, p. 95 to 101).

However, in low alloy steel used for automobile use suspension springs,suppression of corrosion by adjustment of the alloy elements isdifficult. The more sufficiently the corrosion fatigue characteristicscan be improved, the less possible it is to suppress corrosion andpitting. Further, in regions where salt is spread on the roads, thesuspension springs face tough corrosive conditions, so even if adding asmall amount of alloy elements, no effect of suppression of corrosioncan be expected any longer.

Therefore, to improve suspension springs in corrosion fatiguecharacteristics, it is considered effective not to control the corrosionand other surface reactions, but to control the mechanical properties ofthe steel material to improve the fatigue characteristics. To controlthe mechanical properties of steel materials having tempered martensitestructures such as suspension springs, control of the precipitateprecipitating at the time of tempering is important. In particular,spring steel has a relatively large content of carbon, so a large amountof iron carbide inevitably precipitates as well. Further, to obtain highstrength, the steel is tempered at a relatively low temperature, so thesteel material greatly changes in properties due to changes in the stateof the iron carbide precipitating at a low temperature.

Methods for analysis of the precipitation and transition behavior ofiron carbide in the tempering process of spring steel using differentialscanning calorimetry (DSC) have been reported (Nagao Mamoru et al.,“Evaluation of Tempering Behavior of Si-containing Medium-Carbon SteelUsing DSC”, CAMP-ISIJ, vol. 17, 2004, the Iron and Steel Institute ofJapan, p. 359 to 362). However, the relationship between theprecipitation and transition behavior of iron carbide and the mechanicalproperties of steel materials is not described.

Further, as technology for improving the delayed fracturecharacteristics of high strength springs by the control of theprecipitates, methods of making the structure of the spring use steelwire finer and controlling the amount of undissolved carbides have beenproposed (for example, Japanese Patent No. 3764715 and Japanese PatentPublication (A) No. 2006-183137). This technology is technologyeffective for suppressing fracture and improving toughness in a hydrogenenvironment. However, even with these methods, the corrosion fatiguecharacteristics are insufficiently improved. Nothing about fine carbidesprecipitating at the time of tempering is described.

DISCLOSURE OF THE INVENTION

The present invention solves the above-mentioned problems and has as itsobject the provision of high strength spring and high strength springsteel wire superior in corrosion fatigue characteristics suitable forthe suspension spring of an automobile etc. and methods of production ofthe same.

The present invention provides a spring suppressing the precipitation ofcementite (hereinafter sometimes simply indicated as θ) to suppressdeterioration of corrosion fatigue characteristics and causing theprecipitation of epsilon iron carbide (called “ε carbide”) to achieve ahigh strength and a material for the same, that is, a spring use steelwire, and furthermore methods or production suitably controlling therelationship between the temperature and time of tempering and thecomposition of ingredients of the steel. Its gist is as follows:

(1) A high strength spring steel wire characterized by containing, bymass %, C, 0.35 to 0.50%, Si: 1.00 to 3.00%, and Mn: 0.10 to 2.00%,restricting P to 0.015% or less and S to 0.015% or less, having abalance of Fe and unavoidable impurities, and, when raising thetemperature in the range from 50° C. to 600° C. by 0.25° C./s andmeasuring the differential scanning calories, having the only peak ofthe exothermic reaction present at 450° C. or more.

(2) A high strength spring steel wire as set forth in (1) characterizedby further containing, by mass %, Ti: 0.100% or less and B: 0.0010 to0.0100%, restricting N to 0.0100% or less, and having contents of Ti andN satisfying Ti≧3.5N.

(3) A high strength spring as set forth in (1) or (2) characterized byfurther containing, by mass %, one or more of Mo: 0.05 to 1.00%, Cr:0.05 to 1.50%, Ni: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Nb: 0.010 to0.100%, V: 0.05 to 0.20%, and Sb: 0.001 to 0.050%.

(4) A high strength spring characterized by using as a material a highstrength spring steel wire as set forth in any one of (1) to (3).

(5) A high strength spring characterized by containing, by mass %, C,0.35 to 0.50%, Si: 1.00 to 3.00%, and Mn: 0.10 to 2.00%, restricting Pto 0.015% or less and S to 0.015% or less, having a balance of Fe andunavoidable impurities, and, when raising the temperature in the rangefrom 50° C. to 600° C. by 0.25° C./s and measuring the differentialscanning calories, having the only peak of the exothermic reactionpresent at 450° C. or more.

(6) A high strength spring as set forth in (7) characterized by furthercontaining, by mass %, Ti: 0.100% or less and B: 0.0010 to 0.0100%,restricting N to 0.0100% or less, and having contents of Ti and Nsatisfying Ti≧3.5N

(7) A high strength spring as set forth in (5) or (6) characterized byfurther containing, by mass %, one or more of Mo: 0.05 to 1.00%, Cr:0.05 to 1.50%, Ni: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Nb: 0.010 to0.100%, V: 0.05 to 0.20%, and Sb: 0.001 to 0.050%.

(8) A method of production of high strength spring steel wirecharacterized by heating steel wire comprised of the ingredients as setforth in any one of (1) to (3) to 3 to 850 to 1000° C., quenching it,then tempering it under conditions where the tempering temperature T[K],tempering time t[s], and content Si % [mass %] of Si satisfy thefollowing formula 1:

16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)

(9) A method of production of a high strength spring characterized bycold forming steel wire comprised of the ingredients as set forth in anyone of (5) to (7) to a spring shape, heating it to 850 to 1000° C.,quenching it, then tempering it under conditions where the temperingtemperature T[K], tempering time t[s], and content Si % [mass %] of Sisatisfy the following formula 1:

16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)

(10) A method of production of a high strength spring characterized byheating steel wire comprised of the ingredients as set forth in any oneof (5) to (7) to 850 to 1000° C., hot forming it into a spring shape,then quenching it, then tempering it under conditions where thetempering temperature T[K], tempering time t[s], and content Si % [mass%] of Si satisfy the following formula 1:

16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of the results of differential scanning calorimetryof a test piece after quenching and before tempering.

FIG. 2 is an example of the results of differential scanning calorimetryof a test piece after tempering.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors intensively studied the various types of factors affectingthe corrosion fatigue characteristics of high strength suspension springand discovered the following, that is,

(i) If small corrosion pits in the initial stage are formed in thesuspension spring due to corrosion and pitting, fatigue cracks formstarting from these and start to spread.

(ii) In the case of high strength steel such as suspension springs, ifthe toughness is low, the fatigue cracks spread brittlely and the speedof spread of fatigue cracks becomes greater.

(iii) Due to the above reasons of (i) and (ii), improvement of thetoughness of the matrix of the suspension springs leads to longercorrosion fatigue life of the springs and improvement of the corrosionfatigue characteristics.

(iv) To improve the toughness of spring use steel, it is extremelyeffective to add a suitable amount of Si to shift the temperembrittlement temperature region to the high temperature side andsimultaneously reduce the amount of C to a suitable range, and, further,to obtain the targeted strength, perform the tempering at a lowtemperature, that is, at the temper embrittlement temperature region orless.

(v) The iron carbide precipitated at the steel material tempered at thetemper embrittlement temperature region or less is ε carbide. Due tothis, high strength and high toughness can both be achieved. On theother hand, the toughness drops when cementite precipitates.

(vi) In a steel material to which suitable amounts of C and Si are addedand ε carbide is made to precipitate, if adding Ti and B, the toughnesscan be further improved.

(vii) Differential scanning calorimetry (DSC) can be used to identifythe iron carbide precipitated in the steel.

(viii) When a peak of an exothermic reaction due to the transition tocementite is observed by DSC, high strength and high toughness can bothbe achieved. On the other hand, in a steel material where no clearexothermic reaction peak is observed by DSC, excessive precipitation ofθ causes a drop in toughness. Further, in a tempered steel material,when peaks of exothermic reactions due to precipitation of ε carbide andtransition to cementite (θ) are both observed, the yield ratio is lowand the settling characteristic becomes inferior.

Below, the method of identification of iron carbide by DSC will beexplained. DSC is a method of evaluating the precipitation behavior ofmetal materials by detection of the emission of heat and absorption ofheat at the time of raising the temperature.

If measuring a material before tempering, that is, a steel material asquenched, by DSC with a temperature elevation rate of 0.25° C./s, asshown in FIG. 1, a peak of exothermic reaction due to precipitation of εcarbide is observed at the low temperature side, while a peak ofexothermic reaction due to transition to θ is observed at the hightemperature side. Note that in addition, a peak of exothermic reactiondue to breakdown of residual γ has also been reported, but in the caseof spring steel, the amount of residual γ is several %. The peak ofexothermic reaction is also extremely weak, so need not be considered.

The temperatures of the peaks of exothermic reactions due to theprecipitation of ε carbide and the transition of ε carbide to θ changedepending on the steel ingredients. In the case of steel such as springsteel to which Si is added in an amount of 1% or more, a low temperatureside peak is observed at 300° C. or less and a high temperature sidepeak is observed in a 300° C. or more temperature region. Below, thepeak of exothermic reaction due to the precipitation of ε carbideobserved at 300° C. or less will be defined as the “first peak”, whilethe peak of exothermic reaction due to transition of ε carbide to θobserved at 300° C. or more will be defined as the “second peak”.

If measuring by DSC a tempered steel material in which the iron carbideprecipitated is only ε carbide, the precipitation of ε carbide hasalready been completed and the ε carbide changes to θ at the time ofraising the temperature, so, as shown in FIG. 2, the first peak is notobserved. Only the second peak is observed. In the case of such aprecipitated state, both high strength and high toughness can beobtained. On the other hand, if measuring by DSC a tempered steelmaterial with precipitated iron carbide of only θ, the precipitation ofε and transition to θ has already ended, so no clear precipitation peakis observed. In such a state of precipitation, the toughness falls.

Further, if measuring by DSC a tempered steel material where no θprecipitates and ε carbide insufficiently precipitates, in the same wayas a steel material as quenched shown in FIG. 1, the first peak and thesecond peak are both observed. This is for example a case of temperingby a lower temperature than the suitable conditions. The tempering isinsufficient, so the yield ratio is low and the settling characteristicis inferior, so use as a spring is not possible.

Below, the present invention will be explained in detail.

C: C is an element required for obtaining a high strength, so it isnecessary to add 0.35% or more. On the other hand, if adding over 0.50%of C, the toughness falls. Further, if excessively adding C, thetempering temperature for obtaining the desired strength rises, theamount of production of cementite (θ) increases, and it is no longerpossible to obtain both high strength and high toughness, so the upperlimit is preferably made 0.45% or less.

Si: Si is an element effective for strengthening the steel and forimprovement of the settling characteristic of the spring and is animportant element for shifting the temperature at which the ε carbidechanges to θ to the high temperature side. Due to the addition of Si,the temper embrittlement temperature region is shifted to the hightemperature side. If performing the tempering under the conditions offormula 1, the precipitation of ε carbide causes the strength to rise,suppresses the change to θ to avoid temper embrittlement, and enablesthe achievement of both the high strength and high toughness:

16000≦(T−40×[Si %])×(31.7+log t)≦23000  (formula 1)

where, T: tempering temperature (K), t: tempering time (s), and [Si %]:Si content [mass %].

To obtain this effect, Si must be added in an amount of 1.00% or more.On the other hand, if adding Si over 3.00%, decarburization at the timeof rolling or heat treatment of the wire material is assisted, so theupper limit has to be made 3.00%. The preferred range is 1.50% to 2.50%.

Mn: Mn is an element effective for improvement of the hardenability. Ifadded together with Si, the effect is exhibited of suppressing thetransition from ε carbide to θ. To obtain this effect, Mn must be addedin an amount of at least 0.10%, but if added over 2.00%, centersegregation at the time of casting is assisted and the toughness falls.Therefore, the amount of Mn has to be made 0.10 to 2.00% in range. Notethat the preferred range of the amount of Mn is 0.15 to 1.00%.

P, S: P and S are impurities. In particular, P is an element whichsegregates at the old austenite grains and causes embrittlement of thegrain boundaries and lowers the toughness. The upper limits of P and Shave to be limited to 0.015% or less. Further, P and S are preferablyreduced as much as possible. The preferable upper limits are 0.010% orless.

Furthermore, Ti and B are preferably added to restrict the upper limitof N.

Ti: Ti is an element bonding with the N in the steel to causeprecipitation of TiN and thereby fix the N, so contributes to thereduction of the amount of dissolved N. Due to the reduction of theamount of dissolved N, the formation of BN is prevented and the effectof improvement of the hardenability of B is obtained. To fix the N inthe steel, it is preferable to add Ti in an amount of 3.5N or more.However, even if adding over 0.100% of Ti, the effect becomes saturated,so the upper limit should be made 0.100% or less. Further, to suppressthe reduction in toughness due to the coarsening of the TiN and Ti(CN),the upper limit of the amount of Ti is preferably made 0.040% or less.

N is an impurity and is preferably restricted to 0.0100% or less.Further, the smaller the content of N, the smaller the amount ofaddition of Ti that is possible and the smaller the amount of TiNproduced. Therefore, N is preferably reduced as much as possible. Thepreferable upper limit is 0.0060% or less.

B: B is an effective element for improvement of the hardenability ofsteel when added in a fine amount. It also has the effect of segregatingat the old austenite grain boundaries to strengthen the crystal grainboundaries and improve the toughness. In particular, when B is added tosteel containing the amount of C and the amount of Si in the range ofthe present invention, there is the effect of the further improvement ofthe toughness, so addition of 0.0010% or more is preferable. On theother hand, even if adding B in over 0.0100%, the effect becomessaturated. The preferred range of the amount of B is 0.0010 to 0.0030%.Note that to obtain the effect of addition of B, it is preferable toreduce the amount of dissolved N to prevent the formation of BN.Therefore, restriction of the amount of N and addition of Ti areextremely effective.

Furthermore, one or more types of elements of Mo, Cr, Ni, and Cucontributing to the improvement of the hardenability may be selectivelyincluded

Mo: Mo is preferably added in an amount of 0.05% or more to obtain theeffect of improvement of the hardenability, but if added over 1.00%, thealloying cost becomes large and the economicalness is sometimesimpaired. Therefore, the content of Mo is preferably made 0.05 to 1.00%in range. A more preferable range is 0.10 to 0.50%.

Cr: Cr is preferably added in an amount of 0.05% or more to obtain theeffect of improvement of the hardenability, but if added over 1.50%, thetoughness is sometimes impaired. Therefore, the content of Cr ispreferably made 0.05 to 1.50% in range. A more preferable range is 0.10to 0.80%.

Ni: Ni is preferably added in an amount of 0.05% or more to obtain theeffect of improvement of the hardenability, but if added over 1.00%, thealloying cost becomes large and the economicalness is sometimesimpaired. Therefore, the content of N is preferably made 0.05 to 1.00%in range. A more preferable range is 0.10 to 0.50%.

Cu: Cu is preferably added in an amount of 0.05% or more to obtain theeffect of improvement of the hardenability, but if added over 1.00%, thehot ductility falls, the formation of cracks, scratches, etc. at thetime of continuous casting and hot rolling is assisted, and theproducibility of the steel is sometimes impaired. Therefore, the contentof Cu is preferably made 0.05 to 1.00% in range. The preferable range is0.10 to 0.50%.

Furthermore, one or both of Nb and V, contributing to the increasedfineness of the austenite crystal grains, may be included.

Nb: Nb is preferably added in an amount of 0.010% or more to obtain theeffect of improvement of toughness by the increased fineness of thestructure, but even if added over 0.010%, the effect is saturated.Therefore, the content of Nb is preferably 0.010 to 0.100% in range. Amore preferable range is 0.015 to 0.040%.

V: V is preferably added in an amount of 0.05% or more to obtain theeffect of improvement of toughness by the increased fineness of thestructure, but even if added over 0.20%, the effect is saturated.Therefore, the content of V is preferably 0.05 to 0.20% in range. A morepreferable range is 0.10 to 0.15%.

If decarburization occurs at the surface of the spring and spring usesteel wire, the fatigue strength sometimes falls, so Sb may be added tosuppress the decarburization.

Sb: Sb is an element precipitating at the surface of the steel materialand suppressing the decarburization at the time of heating for hotrolling, at the time of cooling after rolling, at the time of heatingfor quenching, etc. To obtain the effect of suppression ofdecarburization, it is preferable to add Sb in an amount of 0.001% ormore, but if added over 0.050%, the hot workability and cold workabilitysometimes deteriorate. Therefore, the content of Sb is preferably made0.001 to 0.050% in range. A more preferable range is 0.002 to 0.020%.

In the present invention, the amount of Al is not defined, but it isalso possible to add Al as a deoxidizing element. Al is also an elementforming a nitride and making the austenite crystal grains finer andcontributes to the improvement of toughness through the increasedfineness of the structure. When using Al for deoxidation, usually 0.010to 0.100% is included. Further, when desiring to suppress formation ofAl-based inclusions, Si, Mn, etc. may be used for deoxidation withoutadding Al.

Iron carbide: To obtain a high strength spring steel and high strengthspring superior in corrosion fatigue characteristics, it is necessary toform ε carbide and suppress the transition to cementite (θ). ε carbideis a finer iron carbide compared with θ and is extremely effective forimprovement of strength and has little detrimental effect on toughness.The suitably tempered high strength spring steel and high strengthspring of the present invention have ε carbide and are suppressed intransition to θ so are excellent in corrosion fatigue characteristics.

The iron carbide of the high strength spring and high strength springsteel of the present invention can be identified by the later explaineddifferential scanning calorimetry.

Differential scanning calorimetry: In differential scanning calorimetry,the temperature elevation rate is important. The iron carbide of thehigh strength spring and high strength spring steel of the presentinvention is identified by a temperature elevation rate of 0.25° C./s.When the range of 50° C. to 600° C. is measured by this temperatureelevation rate, the high strength spring and high strength spring steelof the present invention, as shown in FIG. 2, exhibit an exothermicreaction of only the second peak at 450° C. or more. In this case, it ispossible to judge that the ε carbide in the steel changes to cementiteduring DSC measurement. That is, when the temperature of the exothermicpeak observed is only 450° C. or more, sufficient ε carbide is alreadyformed in the steel and transition to θ is suppressed, so high strengthand high toughness can both be achieved.

On the other hand, when a clear exothermic reaction is not shown, it isjudged that the transition to θ has been completed. In this case,excessive θ is being formed in the steel, so the spring and spring steelfall remarkably in toughness.

Further, the temperature of the second peak changes due to thecomposition of ingredients of the steel, in particular the amount of Si.When the amount of Si is small, if less than 450° C., there is sometimesa second peak. With tempering, there is easy transition to θ. In steelwith a second peak of less than 450° C., θ is excessively formed aftertempering, so the toughness falls. Note that in steel with a temperatureof the second peak of less than 450° C., even if performing thetempering under suitable conditions, part of the ε carbide changes to θ,so if compared with steel having a temperature of the second peak of450° C. or more, the height of the second peak becomes lower.

In the case of a steel material as quenched or insufficient formation ofε carbide, as shown in FIG. 1, a first peak and second peak ofexothermic reaction are exhibited. For this reason, when exhibiting boththe peak of the exothermic reaction accompanying a precipitationreaction of ε carbide and the peak of exothermic reaction when the εcarbide changes to θ, the tempering is insufficient and the ε carbideinsufficiently precipitates, so the yield ratio falls.

Quenching conditions: The heating temperature of quenching of the springand spring use steel wire is made 850° C. or more to make the structureaustenitic, but if over 1000° C., coarsening of the austenite crystalgrains is invited. Therefore, it is necessary to make the heatingtemperature at the quenching 850 to 1000° C. in range. The preferablerange is 900° C. to 990° C. Note that the heating may be performed bythe method of furnace heating, high frequency induction heating, etc.The heating time is usually 5 to 3600 s. It is also possible to heat thespring use steel wire to form it hot to a spring shape and performcooling by quenching (so-called “hot formed spring”). The method ofquenching cooling may be oil cooling, water cooling, etc. Due to thequenching, a mainly martensite structure is obtained.

Tempering conditions: To obtain both high strength and high toughnessafter quenching, tempering is performed under conditions of formula 1:

16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)

where, T: tempering temperature (K), t: tempering time (s), [Si %]: Sicontent [mass %].

When (T−40×[Si %])×(31.7+log t) is less than 16000, the tempering isinsufficient, the yield ratio is low, and the settling characteristic ofthe spring falls. If over 23000, cementite (θ) precipitates and thetoughness falls. Note that the term of the content of Si in the formulaconsiders the effect of Si in making the transition from ε carbide to θshift to the high temperature, long time side. The preferred range ofthe tempering conditions is:

18000≦(T−40×[Si %])×(31.7+log t)≦22000

Note that the cooling after tempering may be performed by either aircooling or water cooling and is not particularly limited.

Note that it is also possible not to form the spring hot, but to performthe quenching and tempering in the state of the rod shape to obtainspring use steel wire, form this into a spring cold, then perform stressrelief annealing. The springs produced by hot forming and cold formingare both shot peened, painted, set, and otherwise processed for use assuspension springs.

EXAMPLE 1

Below, examples will be used to further explain the present invention.

Converter produced steels having the compositions shown in Table 1 wereproduced by continuous casting and, in accordance with need, soaked anddiffusion treated and cogged to obtain 162 mm square rolled materials.Next, hot rolling was performed to obtain wire material shapes ofdiameters of 13 mm. These were annealed in accordance with need, thencold drawn, then cut to predetermined lengths to obtain rods.

Next, these were heated in a heating furnace to 980° C. and held therefor 30 minutes, then the rods were wrapped around a drum hot to formthem into predetermined spring shapes which were then immediatelyimmersed in oil for quenching. Further, the material for obtaining thetensile test piece and Charpy impact test piece was quenched as in a rodshape without forming it into a spring shape.

Next, a spring shaped material and rod material were tempered under theconditions shown in Table 2. The heating method in the tempering wasmade furnace heating or high frequency induction heating. A tensile testpiece with an 8 mm diameter of the parallel part and a U-notch testpiece based on JIS Z 2242 (subsize, width 5 mm) were fabricated from atempered rod and used for a tensile test and Charpy impact test.

In the tensile test, the tensile strength and the 0.2% yield strengthwere measured and the yield ratio was found. The tensile strength andyield ratio preferable as a suspension spring were defined as 1800 MPaor more and 0.85 or less. If satisfying these, the strength and settlingcharacteristic are judged to be good when used as a suspension spring.The test temperature in the Charpy impact test was made 20° C. Further,a sample with an impact value of 75 J/cm² or more was deemed good. Dueto this, it was judged that the corrosion fatigue characteristics wereimproved.

Further, a test piece for differential scanning calorimetry (length3×width 3×thickness 1 mm) was taken from the spring shaped material. TheDSC curve was measured under measurement conditions of the differentialscanning calorimetry of an atmospheric gas: N₂ (30 ml/min), measurementtemperature range: 50 to 600° C., cell: aluminum, and reference: α-Al₂O₃and with a temperature elevation rate of 0.25° C./s to find thetemperatures of the exothermic peaks. These test results are showntogether in Table 2. Note that the “-” of the DSC exothermic peak ofTable 2 indicates that no clear peak is seen. Further, the rod materialwas cold formed: into a spring shape, then similarly subjected tomechanical tests and measured for DSC curve. It was confirmed thatcharacteristics equal to the results shown in Table 2 are obtained.

As shown in Table 2, the steel materials of the Manufacturing Nos. 1 to10 of the present invention are superior to the comparative examples intoughness and characteristics as suspension springs. On the other hand,Manufacturing No. 11 has an amount of C over the range of the presentinvention, so a high impact value is not obtained. Manufacturing No. 12has an amount of C not satisfying the range of the present invention, sothe tensile strength becomes lower as quenched and the tensile strengthas a suspension spring cannot be obtained. Manufacturing Nos. 13 to 15have amounts of Si not satisfying the range of the present invention, sothe temperatures of the second peaks are low, θ forms in the steels, andhigh impact values cannot be obtained.

Manufacturing No. 16 has an amount of Mn over the range of the presentinvention, so a high impact value cannot be obtained. Nos. 17 and 19have high tempering temperatures and has tempering conditions over theranges of the present invention, so cementite precipitates, theexothermic peaks in DSC are not clear, and high impact values cannot beobtained. Manufacturing No. 18 has a low tempering temperature andtherefore tempering conditions not reaching the range of the presentinvention, so the tempering is insufficient and ε carbides areinsufficiently formed, so even at less than 300° C., an exothermic peakis caused, the yield ratio is low, and use as a suspension spring is notpossible.

TABLE 1 Ingredient (mass %) 3.5 x C Si Mn P S N Ti B Mo Cr Ni Cu Nb V Sb[N %] Remarks A 0.40 1.99 0.22 0.005 0.005 0.0040 0.035 0.0025 0.35 — —— — — — 0.014 Inv. B 0.40 1.73 0.40 0.004 0.002 0.0035 0.026 0.0015 0.35— — — — — — 0.012 ex. C 0.40 2.00 0.44 0.001 0.015 0.0051 0.033 0.00180.25 — 0.05 0.05 — 0.05 — 0.018 D 0.40 1.75 0.51 0.015 0.001 0.00250.035 0.0010 0.25 0.05 — — 0.010 — — 0.009 E 0.35 3.00 0.88 0.002 0.0020.0063 — — — — — — — — — 0.022 F 0.50 1.00 0.32 0.003 0.002 0.0043 — —1.00 — — — — — — 0.015 G 0.38 2.20 2.00 0.002 0.004 0.0015 — — — — — —0.100 — — 0.005 H 0.45 1.50 0.10 0.006 0.003 0.0028 0.100 — — — 1.00 — —— — 0.010 I 0.42 1.90 0.15 0.005 0.006 0.0029 0.020 0.0022 — 1.50 0.501.00 — 0.20 — 0.010 J 0.40 2.01 0.25 0.005 0.004 0.0035 0.032 0.00200.36 — — — — — 0.0050 0.012 K 0.55 1.90 0.45 0.003 0.006 0.0045 0.0340.0015 0.37 — — — — — — 0.016 Comp. L 0.32 1.85 0.51 0.008 0.005 0.00300.031 0.0023 0.29 — — — — — — 0.011 ex. M 0.41 0.77 0.55 0.007 0.0030.0033 0.029 0.0019 0.33 — — — — — — 0.012 N 0.42 0.93 0.45 0.009 0.0050.0040 0.030 0.0021 0.33 — — — — — — 0.014 O 0.45 0.95 0.77 0.010 0.0030.0035 — — 0.40 0.76 0.05 0.05 0.015 — — 0.012 P 0.39 1.53 2.31 0.0060.003 0.0039 — — — — — — — — — 0.014 “—” in the elements mean noaddition. The underlines in the table show outside the range of thepresent invention.

TABLE 2 Tempering conditions Heating Heating DSC exothermic peak TensileImpact Man. Steel temp. time (T − 40 × [Si %]) × First peak Second peakstrength Yield value No. no. [Si %] ° C. s Cooling (31.7 + logt) ° C. °C. MPa ratio J/cm² Remarks 1 A 1.99 375 3600 Air 20040 — 480 1954 0.91118  Inv. 2 B 1.73 360 1800 Air 19708 — 469 1923 0.89 120  ex. 3 C 2.00390 1800 Air 20379 — 481 1999 0.93 117  4 D 1.75 410 30 Water 20338 —471 1910 0.94 103  5 E 3.00 450 3600 Air 21260 — 521 1899 0.95 97 6 F1.00 410 3600 Air 22670 — 451 2123 0.95 91 7 G 2.20 400 3600 Air 20625 —488 1813 0.94 96 8 H 1.50 400 3600 Air 21612 — 461 1971 0.93 94 9 I 1.90430 5 Air 20314 — 476 1944 0.92 101  10 J 2.01 375 3600 Air 20011 — 4801955 0.91 119  11 K 1.90 375 3600 Air 20167 — 477 1956 0.93 45 Comp. 12L 1.85 350 3600 Air 19356 — 474 1768 0.85 89 ex. 13 M 0.77 395 3600 Air22465 — 432 1932 0.95 53 14 N 0.93 345 3600 Air 20477 — 438 1926 0.93 5615 O 0.95 350 3600 Air 20625 — 448 1997 0.93 55 16 P 1.53 375 3600 Air20688 — 462 1933 0.94 49 17 A 1.99 460 3600 Air 23036 — — 1803 0.95 4818 A 1.99 300 3600 Air 17395 230 480 2166 0.66 71 19 F 1.00 440 3600 Air23727 — — 1947 0.95 42 Underlines in Steel No., [Si %], (T − 40 × [Si%]) × (31.7 + logt), DSC exothermic peak temperature indicate outsidethe range of the present invention. Underlines in tensile strength,yield ratio, and impact value indicate target not yet achieved.

EXAMPLE 2

Wire materials of diameters of 13 mm of Steel Nos. A to J shown in Table1 hot rolled in the same way as Example 1 were used to evaluate theeffect of suppression of decarburization by the addition of Sb. The wirematerials were straightened, then ground at their outer circumference toremove the effects of the initial surface layers and obtain 12φ rod testpieces. The test pieces were heated to 870° C., was then held for 30minutes, then were transferred to a 750° C. furnace, held there for 60minutes, then air-cooled. The heat treatment was all performed in theatmosphere. The heat treatment conditions were heat treatment conditionsvery conducive to decarburization. After the heat treatment, the Ccross-sections of the rod test pieces were cut, polished, and corrodedby nitral and the depths of the decarburized layers of the surfaces weremeasured.

The results are shown in Table 3. Steel A has no Sb added, while Steel Jhas substantially the same ingredients as Steel A, but has Sb added. Asclear from Table 3, due to the addition of Sb, the depth of thedecarburization layer becomes half or less and decarburization issuppressed.

TABLE 3 Steel Heat treatment Heat treatment Decarburization no. [Sb %]conditions atmosphere depth (μm) Remarks A — 870° C. × 30 min Air 110Inv. ex. J 0.0050 → 750° C. × 60 min 40 → air-cooling

INDUSTRIAL APPLICABILITY

According to the high strength spring and high strength spring steelwire with superior corrosion fatigue characteristics and methods ofproduction of the same of the present invention, it becomes possible toreduce the size and lighten the weight of a suspension spring andgreatly contribute to the improvement of the fuel economy andimprovement of the performance of an automobile etc.

1. A high strength spring steel wire characterized by containing, bymass %, C, 0.35 to 0.50%, Si: 1.00 to 3.00%, and Mn: 0.10 to 2.00%,restricting P to 0.015% or less and S to 0.015% or less, having abalance of Fe and unavoidable impurities, and when raising thetemperature in the range from 50° C. to 600° C. by 0.25° C./s andmeasuring the differential scanning calories, having the only peak ofthe exothermic reaction present at 450° C. or more.
 2. A high strengthspring steel wire as set forth in claim 1 characterized by furthercontaining, by mass %, Ti: 0.100% or less and B: 0.0010 to 0.0100%,restricting N to 0.0100% or less, and having contents of Ti and Nsatisfying Ti≧3.5N
 3. A high strength spring as set forth in claim 1characterized by further containing, by mass %, one or more of Mo: 0.05to 1.00%, Cr: 0.05 to 1.50%, Ni: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Nb:0.010 to 0.100%, V: 0.05 to 0.20%, and Sb: 0.001 to 0.050%.
 4. A highstrength spring characterized by using as a material a high strengthspring steel wire as set forth in claim
 1. 5. A high strength springcharacterized by containing, by mass %, C, 0.35 to 0.50%, Si: 1.00 to3.00%, and Mn: 0.10 to 2.00%, restricting P to 0.015% or less and S to0.015% or less, having a balance of Fe and unavoidable impurities, andwhen raising the temperature in the range from 50° C. to 600° C. by0.25° C./s and measuring the differential scanning calories, having theonly peak of the exothermic reaction present at 450° C. or more.
 6. Ahigh strength spring as set forth in claim 5 characterized by, furthercontaining, by mass %, Ti: 0.100% or less and B: 0.0010 to 0.0100%,restricting N to 0.0100% or less, and having contents of Ti and Nsatisfying Ti≧3.5N
 7. A high strength spring as set forth in claim 5,characterized by further containing, by mass %, one or more of Mo: 0.05to 1.00%, Cr: 0.05 to 1.50%, Ni: 0.05 to 1.00%, Cu: 0.05 to 1.00%, Nb:0.010 to 0.100%, V: 0.05 to 0.20%, and Sb: 0.001 to 0.050%.
 8. A methodof production of high strength spring steel wire characterized byheating steel wire comprised of the ingredients as set forth in claim 1to 850 to 1000° C., quenching it, then tempering it under conditionswhere the tempering temperature T[K], tempering time t[s], and contentSi % [mass %] of Si satisfy the following formula 1:16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)
 9. A method of production ofa high strength spring characterized by cold forming steel wirecomprised of the ingredients as set forth in claim 5 to a spring shape,heating it to 850 to 1000° C., quenching it, then tempering it underconditions where the tempering temperature T[K], tempering time t[s],and content Si % [mass %] of Si satisfy the following formula 1:16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)
 10. A method of productionof a high strength spring characterized by heating steel wire comprisedof the ingredients as set forth in claim 5 to 850 to 1000° C., hotforming it into a spring shape, then quenching it, then tempering itunder conditions where the tempering temperature T[K], tempering timet[s], and content Si % [mass %] of Si satisfy the following formula 1:16000≦(T−40×[Si %])×(31.7+log t)≦23000  (1)